In recent years, a silicon thin film prepared by vapor deposition technique has achieved widespread acceptance in the various technical fields. To prepare such a silicon thin film, silicon undergoes heating, such as resistance heating, induction heating, and electron-beam heating. In the electron-beam heating, the application of heat is localized to the surface of silicon as an evaporation source. Therefore, the electron-beam heating effectively increases evaporation speed compared to the resistance heating and the induction heating where the whole silicon as the evaporation source has to be heated.
In the electron-beam heating, however, a small region of the surface exposed to electron beams undergoes fast-paced heating. Silicon as the evaporation source often contains water and low-boiling impurities, such as organic material, phosphorus, calcium, and aluminum. For example, metal grade silicon made by reducing natural silicon oxide contains a large amount of aluminum.
When such metal grade silicon is used for the evaporation source, sharply accelerated evaporation of low-boiling impurities allows the silicon to spatter and attach on the surface of a thin film to be processed. This affects the quality of the film, such as in thickness and composition. Besides, the evaporation substrate can suffer damage due to the silicon spatter. In particular, during continuous feeding of solid silicon or a melting process of solid silicon in batch feeding, namely when solid silicon remains in the evaporation source, a sharp increase in temperature of heating easily causes the silicon spattering. This is because the fact that solid silicon has a density smaller than silicon-melt and accordingly solid silicon tends to stay on the surface.
To suppress the silicon spattering due to fast-paced heating in the melting process, it is effective that the vapor pressure on aluminum impurities is set lower than the furnace pressure. When a dilute solution is heated, the vapor pressure of the solvent and the solute of the solution can be determined on Raoult's law and/or Henry's law. Similarly, when solid silicon that contains impurities is heated, decreasing the concentration of low-boiling impurities allows the saturated vapor pressure thereof to be lower, suppressing vigorous evaporation.
Solid solubility of aluminum to solid silicon depends on the temperature; in the range from 1200° C. to 1400° C., the solubility of aluminum to solid silicon is approx. 500 ppm (by weight). That is, aluminum having a concentration equal to or lower than 500 ppm is soluble in silicon, and it is rarely deposited on the grain boundary, which suppresses vigorous evaporation of aluminum when melting. On the other hand, when aluminum with higher concentration is included in silicon, indissoluble aluminum gathers on the grain boundary, whereby a highly aluminum-concentrated region is formed. Therefore, in a case where silicon as the evaporation source has an aluminum concentration higher than 500 ppm, aluminum vigorously evaporates in the electron-beam heating. This is because the fact that silicon-melt having a high aluminum concentration is formed locally on the aluminum-concentrated grain boundary. In this way, vigorous evaporation of aluminum allows silicon to easily spatter.
For producing semiconductors and solar cells, silicon requires to have purity of 99.99% or higher. When remnants of silicon used above (i.e., scrap silicon) are employed for the evaporation source, the silicon spattering is suppressed. However, there is a great demand for such a highly purified silicon, and accordingly, it is hard-to-get and highly expensive material.
Under these circumstances, to obtain a low-cost evaporation source, a suggestion—where silicon purified by a metallurgical method (the so-called metal grade silicon) is prepared as a raw material—has been made. Having a purity of approx. 98%, metal grade silicon contains low-boiling impurities such as phosphorus, calcium, and aluminum as mentioned above.
On the other hand, as a purification method to obtain silicon having impurities lower than 1 ppm, i.e., having purity high enough for producing solar cells, some suggestions have been made. Specifically, Patent documents 1 and 2 show well-known methods where silicon is purified by heating in the atmosphere under a reduced pressure.
According to the methods shown in Patent documents 1 and 2, the purification is carried out under high vacuum condition (not greater than 10 Pa). In such an atmosphere, the low-boiling impurities evaporate at a high speed, which easily invites silicon spattering in the purifying process. Therefore, the purification carried out under high vacuum condition is inappropriate for purifying metal grade silicon containing aluminum of 1000 ppm or higher.
Patent document 3 shows a purifying method to obtain silicon with purity of 98 to 99%. According to the method, impurities are removed by an elution process with the use of acid. The method, however, cannot reduce the concentration of aluminum impurity to 500 ppm or lower.
As described above, in the case where metal grade silicon containing aluminum of 1000 ppm or higher is used as a raw material, conventional methods have brought little success in reducing the concentration of aluminum impurity to 500 ppm or less with no use of chemical technique.    Patent document 1: Japanese Patent Unexamined Publication No. H06-227808    Patent document 2: Japanese Patent Unexamined Publication No. 2006-232658    Patent document 3: Japanese Patent Unexamined Publication No. H05-33070